Carbon fiber-reinforced plastics (CFRP) are applied widely to various sporting goods such as golf club shafts and fishing rods, airplanes, automobiles, pressure vessels, and the like, by making good use of their characteristics such as high strength and light weight, and are expected for future applications. The fiber-reinforced plastics are molded into a target molded product by a general molding method, including a contact pressure molding method such as a hand lay-up method and a spray-up method, a filament winding (FW) method, a drawing method, and a continuous molding method such as a continuous lamination method. A matrix resin used is a thermosetting resin such as an epoxy resin. To increase a binding force with a matrix resin, a sizing agent appropriate for the matrix resin is applied to the surface of reinforcing fibers (above, Non-Patent Document 1).
As conventional techniques, Patent Document 1 proposes the use of a sizing agent for carbon fibers that contains an acrylic group and an epoxy group. Patent Document 2 proposes subjecting the surface of carbon fibers to ozone oxidation before application of a sizing agent. Patent Documents 3 and 4 propose the use of a sizing agent for carbon fibers that contains an epoxy group.